1. Technical Field
The present disclosure relates to a method for forming a vertical thin-film lithium-ion type battery.
2. Discussion of the Related Art
Lithium-ion type batteries have the advantage of comprising a solid non-flammable electrolyte which further has a good ion conductivity over a wide range of temperatures. Such batteries could advantageously be used in mobile electronic devices such as portable phones or computers.
To form thin-film lithium-ion batteries, typically batteries having 2.5×2.5-cm2 dimensions, the use of techniques of sputtering through a shadow mask is known. Such techniques comprise placing a shadow mask above a support and sputtering, through this mask, the different layers forming the battery.
However, for a large-scale production of thin-film lithium-ion type batteries, the use of shadow masks may necessitate a relatively high cost. Indeed, for each sputtering using a mask, the sputtered component also deposits on the mask. Thus, between each use of the mask, it is necessary to remove and to recycle the different layers deposited on this mask.
Techniques for forming batteries by sputtering through a shadow mask also have the advantage of not being adapted to form batteries having smaller dimensions (with a side length smaller than 3 mm), mask alignment problems appearing for such dimensions. Now, such batteries could advantageously be used as backup batteries to save key data in memories in case of a failure of a main battery.
To form small lithium-ion type batteries, it could be devised to adapt lithography techniques which are well known and controlled in the field of integrated circuits. However, such techniques entail relatively high costs if they are applied to large substrate surface areas. Further, lithography methods require the use of resins which are removed by wet processings (generally solvent-based aqueous mixtures) which would cause chemical reactions with the very reactive lithium-based layers of the battery. Such techniques are thus complex to implement for the manufacturing of lithium-ion type batteries.
FIG. 1 corresponds to FIG. 4 of published U.S. patent application No. US-2011-0076567-A1 of the applicant illustrating a lithium-ion type battery structure at an intermediary step of its manufacturing.
The structure comprises a conductive substrate 10 having an insulating material layer 12 formed thereon. As an example, substrate 10 may have a thickness ranging between 500 and 800 μm and may be made of doped silicon or again of a metal. Insulating layer 12 may be made of silicon oxide and have a thickness ranging between 5 and 30 μm.
A stack of the different layers forming a lithium-ion type battery is deposited in an opening formed in layer 12. This stack comprises the following layers:                a first layer 18 forming a cathode collector; this layer may be made of titanium, of tungsten, of molybdenum, of tantalum, of platinum, or of an alloy or a stack of these materials and have a thickness ranging between 100 and 500 nm;        a second layer 20 forming the battery cathode, made of a material such as LiTiOS (lithium titanium oxysulphide), LiCoO2 (cobalt and lithium oxide), or LiFePO4 (iron and lithium phosphate), having a thickness ranging between 1 and 10 μm; more generally, layer 20 may be made of any lithium insertion material usable as a cathode in lithium-ion type batteries;        a third layer 22 forming the electrolyte of the battery, for example, formed of LiPON (Lithium Phosphorus Oxynitride) or of any material capable of forming a solid lithium-ion battery electrolyte; third layer 22 has a thickness ranging between 1 and 3 μm;        a fourth layer 24 forming the battery anode, for example, made of silicon, germanium, carbon, or of a stack or an alloy of these materials; the fourth layer has a thickness ranging between 10 and 800 nm;        a fifth layer 26 forming an anode collector or a seed layer for the anode collector; this layer may be formed of a stack of titanium and copper; layer 26 may have a thickness ranging between 100 and 300 nm; it is possible not to provide layer 26 if the material forming the anode collector layer is capable of being directly formed on the battery anode.        
Thus, the stack of layers 18 to 26 forming the active portion of the lithium-ion type battery may have a thickness ranging between 5 and 15 μm.
To achieve the structure of FIG. 1, successive conformal depositions have been performed, after which a planarization or polishing has been performed to remove all the layers located above the upper surface of the portions of insulating material 12 and thus expose this insulating material. The planarization is performed by chem.-mech. polishing (CMP). It should however be ascertained to use non-aqueous planarization compounds which do not react with the materials of layers 20 and 22. This planarization step may be particularly difficult to implement.
Thus, there is a need for a method enabling to form thin-film lithium-ion type batteries, this method being relatively inexpensive to implement and adapted to the presence of the very reactive materials forming these batteries.